1. Field of the Invention
The present invention relates to an apparatus for preventing leakage of a weak signal from a speaker, and more particularly, to an apparatus for preventing leakage of a weak signal from a speaker, which passes a normal speaker drive signal coming from an audio system through speaker signal lines and blocks a weak signal generated by the sounds around the speaker.
2. Discussion of Related Art
A speaker, which is a device for generating sound, can be maliciously used for eavesdropping in some situation, since it has a structure similar to a microphone. FIGS. 1A and 1B are schematic diagrams showing structures of a speaker 100a and a microphone 100b respectively, and FIG. 1C is a schematic diagram showing a structure of a broadcast speaker for announcement, 100c. 
Referring to FIG. 1A, the conventional speaker 100a has a structure including a voice coil 120a surrounding a permanent magnet 110a and a diaphragm 130a attached to the voice coil 120a. When an output current of a signal generator D1 is transmitted through speaker signal lines 140a and flows to the voice coil 120a, the diaphragm 130a moves according to the electromagnetic force by the Lorentz force law, thereby generating sound.
Referring to FIG. 1B, the microphone 100b has the similar structure as the speaker 100a. When the diaphragm 130b is moved by microphone-around sounds, a current flows in the voice coil 120b according to electromagnetic induction. The current is transmitted to a signal receiver D2 through the microphone signal lines 140b. 
As described above, the speaker 100a have basically the similar structure as the microphone 100b, so a current may be generated by speaker-around sounds and flows in the voice coil 120a of the speaker 100a, as in the microphone 10b. In this case, when the current flowing in the voice coil 120a is leaked through the speaker signal lines 140a, the conversation around the speaker may be eavesdropped.
Specifically, when broadcast speakers for announcement are installed at multiple positions, conversations around all speakers connected to the common signal lines may be eavesdropped simultaneously if a person branches off the common signal lines.
Referring to FIG. 1C, the broadcast speaker 100c generally includes a transformer 160 which is used to minimize the transmission power loss but may make eavesdropping easy because it amplifies a weak signal of the voice coil 120a. 